A digital/analog amplifier of this kind, for example, is commonly used as a D-class amplifier for audio applications, especially embedded applications. This is due to the fact that they have very high rates of efficiency.
The signal to be amplified is pulse-width modulated. The control circuit produces drive signals for the transistors of the inverter as a function of the modulated signal. The output of the inverter is applied to the input of the filter, and a sine signal is recovered at the output of the filter. The output of the amplifier, formed by the output of the filter, is connected to the terminals of a load that is a resistor formed by the impedance of an audio loudspeaker, for example.
Preferably, to obtain optimum efficiency, a differential amplifier is provided. This differential amplifier comprises two arms like the one described above. The output of each arm is connected to a terminal of the load.
To prevent short-circuiting of the power supply of the amplifier, the transistors of the power inverter should not be on simultaneously. For this reason, a lag is introduced between the power-off command for one of the transistors of the inverter and the power-on command for the other transistor. This lag is commonly called a dead time.
While this lag has the advantage of preventing short-circuiting of the power supply, it has the drawback of introducing a distortion into the output signal at the terminals of the load of the amplifier in the form of odd-parity harmonics. The distortion increases with the dead time and with the frequency of the switch-over of the transistors of the power inverters of the amplifier. Reducing the dead time reduces the distortion that it causes at the output of the amplifier. However, the dead time cannot be eliminated, and the distortion that it introduces cannot be eliminated either.
This phenomenon of distortion has been studied, for example, in the documents titled “Compensating For Dead Time Degradation Of PWM Inverter Waveforms”, R.C. Dodson et al., IEEE Proceedings B, Vol. 137, Issue 2, March 1990, and “The Analysis And Compensation Of Dead-Time Effects In PWM Inverters”, Seung-Gi Jeong et al., Industrial Electronics, IEEE Transactions, Vol. 38, Issue 2, April 1991, pages 108-114. To reduce the distortion due to the dead time, the above two documents introduce a delay in the input signal in the control part of the power amplifier. However, this approach is difficult to implement because it is difficult to apply a delay equal to the dead time, especially when the dead time is small.
It has also been proposed to insert a negative feedback loop into the modulated signal to re-inject a corrective signal as a function of the distortion detected at the output. This approach is very efficient but providing a negative feedback loop is difficult and costly.